From blackmagic design website.
Does the Blackmagic eGPU work with all macOS computers?

The Blackmagic eGPU will work with all Thunderbolt 3 based Apple computers that are running macOS 10.13.6 or later. This includes MacBook Pro computers from the 2016 model year and later, 2017 iMac and iMac Pro.

Can I use an Apple Thunderbolt 3 to Thunderbolt 2 adapter with an older Thunderbolt 2 computer?

No, the Blackmagic eGPU won't work with Thunderbolt 2 computers as they don't support the speed required for good eGPU performance.